This application is a Continuation of International Application PCT/F100/00173 filed Mar. 3, 2000 which designated the U.S. and was published under PCT Article 21(2) in English.
The invention relates to an electrically conductive thermoplastic elastomer comprising a substantially electrically non-conductive matrix material and a filler part.
The invention further relates to a sealing and/or an EMI shielding product.
The continuous increase in electronic equipment has created a need, on one hand, to shield the equipment from electromagnetic interference caused by other electronic equipment and, on the other hand, to reduce the external interference the equipment itself causes. It is especially important to shield equipment whose operational malfunctions may cause physical injury, such as medical equipment and the control and navigational equipment in aircraft. The term EMI (electromagnetic interference) shielding is generally used to refer to the electromagnetic shielding of equipment. The frequency range of the electromagnetic interference in question is typically 100 MHz to 10 GHz.
EMI shielding can be implemented by a casing made of electrically conductive material. The specific resistance of the material should be approximately 1 xcexa9.cm or less. Several electrically conductive plastic composites have been developed for EMI shielding casings, the composites usually comprising a thermoplastic plastic or plastic compound with one or more conductive fillers, such as metal powder, metal fiber or carbon black. The EMI shielding level of casing composites is sufficient most of the time, but the joints and seams of the casing parts cause problems. A good shielding ability of a casing material does not guarantee a good shielding efficiency, if the sealings or adhesives do not form an electrically conductive cross-link between the different parts of the casing. To solve the sealing problem, various electrically conductive cross-linked and thermosetting elastomers have been developed, in which a substantially electrically non-conductive polymer acting as a matrix material is mixed with metal or metallic particles, carbon, graphite or combinations thereof acting as a conductive filler. It should be mentioned here that the term elastomer refers to a material made of macromolecules, which is characterized by ductility and quick recovery to original shape after the tension is released. In sealing applications, the hardness of the elastomer should preferably be less than 65 Shore A to ensure an easy mounting and appropriate operation. The specific resistance of said prior art electrically conductive elastomers with metal fillers are typically in the range of 10xe2x88x923 to 10xe2x88x921 xcexa9.cm and with carbon fillers approximately 0.5 xcexa9.cm or more. As stated above, the prior art electrically conductive elastomers are based on cross-linked, for instance silicone-based, matrix materials which require cross-linking to obtain elastic properties and to enable product processibility. Cross-linking requires a lot of energy and time as well as specific cross-linking means, which makes manufacture of products slow and costly.
Prior art also includes electrically conductive thermoplastic elastomers mixed with carbon black as the electrically conductive filler. The specific resistance of these materials is, however, considerably higher than that of materials based on cross-linked elastomers. It should be noted that in this application, the abbreviation TPE is used for electrically conductive thermoplastic elastomers. Publication U.S. Pat. No. 4,321,162, for instance, discloses a TPE comprising an ethylene-copolymer-propylene polymer compound with carbon black. The specific resistance stated for the material is at its lowest 104 xcexa9.cm which is not low enough for EMI applications. The specific resistance can be lowered somewhat by increasing the proportion of carbon black in the material, but then the workability and end-use properties deteriorate substantially.
Publication U.S. Pat. No. 5,736,603 discloses an electrically conductive composite material in which a thermoplastic elastomer is impregnated with electrically conductive fibers. The reported volumetric specific resistance is at its lowest approximately 106 xcexa9.cm which is not enough for EMI shielding. The length of fibers is approximately 10 mm, which means that small-scale products with electrical properties substantially similar in various directions of the product cannot be manufactured of the material. In addition, the manufacturing process of the material comprises several phases making it costly.
It is an object of this invention to provide an improved thermoplastic electrically conductive elastomer and a sealing and/or EMI shielding product.
The thermoplastic electrically conductive elastomer of the invention is characterized in that the molecule-level structure and/or additives of the elastomer is such that the elastomer can be injection-molded and/or extruded and that the filler contains metal whose proportion is so high that the specific resistance of the electrically conductive thermoplastic elastomer is at most 1 xcexa9.cm.
The essential idea of the invention is that the TPE is filled with a metal-containing electrically conductive filler so that the specific resistance of the electrically conductive thermoplastic elastomer is at most 1 xcexa9.cm, preferably at most 0.1 xcexa9.cm. Further, the idea of the invention is that the electrically conductive TPE can be worked using injection-molding and extrusion methods. The idea of a preferred embodiment is that the viscosity of the TPE matrix material is below the viscosity curve xcex7=43625.7xc2x7xcex30.152374xe2x88x921, preferably below xcex7=22197.62xc2x7xcex30.120327xe2x88x921, which equations are based on the general presentation format of a viscosity curve
xcex7=Kxcex3nxe2x88x921,
where xcex7=viscosity [Pas], xcex3=shear rate of the material [sxe2x88x921] and K, n=material-specific constants. The idea of a second preferred embodiment is that the TPE comprises a styrene-ethylene-butylene-styrene copolymer (SEBS) based matrix material to which it is especially easy to mix a filler due to its low viscosity. The idea of a third preferred embodiment is that the TPE comprises a styrene-ethylene-propylene-styrene copolymer (SEPS) based matrix material to which it is also easy to mix a filler due to its low viscosity. The idea of a fourth preferred embodiment is that the filler content is at least 5 volume percent of the entire TPE volume. The idea of a fifth preferred embodiment is that the fill factor of the metal-containing filler is at least 30 volume percent of the entire material volume, which makes it possible to achieve even lower specific resistance values. The idea of a sixth preferred embodiment is that the TPE is filled with at least two electrically conductive fillers with a different particle shape and containing metal, whereby a low TPE specific resistance value can be achieved with a smaller filler content.
The invention provides the advantage that the material provides an excellent EMI shielding efficiency in a finished, for instance injection-molded, product, while retaining the low hardness and other mechanical properties of the TPE at a sufficient level in order to be able to use the material in sealing applications. The manufacturing of the material does not require any special equipment or methods, but only conventional mixing means, for instance a twin-screw extruder or a roll mill, in which the material components are mixed in suitable proportion to each other. The material can be worked in a versatile manner by equipment and methods generally used in plastics industry, such as injection molding, extrusion, thermoforming or any other suitable method. The melt viscosity of the material is sufficiently low to allow the manufacture of high quality products with exacting shapes.